Generally where all or part of a log is not destined to be pulped it is usual to categorise the lumber to be taken by a log break down system into that which is destined to be cut to structural dimensions and that which is to be cut to industrial dimensions. Wood which is sufficient stiffness for structural application is usually broken down into timbers for construction or similar applications.
Stiffness is a lesser concern for generating industrial applications and industrial dimensioned lumber issued in a variety of end uses such as the making of packing cases for example.
The outer wood or sap wood from a log or tree stem is generally of higher stiffness than the core of the log or stem, or heart wood. In general structual application lumber has the highest market value and it is desirable to saw tree stems or logs so as to maximise the value of wood which is cut to structural dimensions. There can be a significant pricing differential per unit value between structural dimensioned lumber and industrial dimensioned lumber. By way of example to a specific instance, we wish to refer to two types of mistake that might be made in a break down system.
New Zealand patent specification 333434 discloses a procedure which enables in the forest (at a skid site) a reasonable determination of the average stiffness of a tree stem and thus logs to be cut therefrom. New Zealand patent specification 331527 discloses a related method appropriate for determining those tree stems and/or logs that are suitable for pulping.
PCT international patent application US98/23921 published as WO 99/44059 discloses a method of lumber break down to maximise the value of the lumber recovered from a log by a system which includes measuring the length of the log and determining a stress wave velocity in the log, to predict an average modulus of elasticity. It is clear that an average velocity for the full log section or tree stem section is utilised.